confused by xrandr man page example

[Felix Miata]

"Enables panning on a 1600x768 desktop while displaying 1024x768 mode on an output called VGA:
       xrandr --fb 1600x768 --output VGA --mode 1024x768 --panning 1600x0"

I get the same visible area and mouse access whether

	--panning 1600x768
or
	--panning 1600x0
while
	--panning 1600x384

seems to constrain access to the left 800.

It seems obvious why x is 1600, but y=0 seems inconsistent with the specification of x.
Are 0 and 768 equally correct for y=768?
-- 
"The wise are known for their understanding, and pleasant
words are persuasive." Proverbs 16:21 (New Living Translation)

 Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!

Felix Miata  ***  http://fm.no-ip.com/